After taking the DX-390's plastic enclosure apart by removing all the black self-tapping screws (don't forget the one down in the battery compartment) and lifting up the single p.c. board by unscrewing all the silver colored screws which hold it down, you will find Q1 near the antenna connector which in my radio was a 2SK152. Finding data on this Sony device is not easy and replacement devices are just as obscure. I get the impression that it is a low Ciss small signal N-JFET with moderate forward transconductance (>5mS)--a 2N4416 would be an excellent choice for replacement from an electrical standpoint. Not having one, I decided to buy a 2N3819 at Radio Shack (part number 276-2035, 99 cents each). Actually, I bought two and used the one with the lowest rds(Vgs=0). You can measure this with a DVM by holding the gate and source together and measuring the resistance from them to the drain. For a JFET, 1/rds ~= gfs. I measured 275ohms, which would correspond to a gfs of 3.63mS, not great, but not bad.
Remove the dead Q1 by sucking the solder off of its p.c. pads with solder wick and measure it with the DVM to make sure it is dead--mine looked like a 31-ohm resistor from drain to source. A good FET would look like a pair of diodes with their anodes connected together at the gate. The gate is the lower terminal looking at the device from the bottom with the flat side to your right; this leads to an IMPORTANT difference in the DX-390's Q1 and the 2N3819 pinout: the source terminal is the same but the gate and drain must be interchanged. This means that when you replace the bad Q1 with a 2N3819, you must insulate the 2N3819 gate lead and twist it with the drain 180 degrees so the drain goes into the middle hole. For insulation, I used 1/8 inch of teflon insulation I removed from some #22 hookup wire. The transistor will stick up more than the original Q1 but this did not seem to cause any problems.
While you're at it, with the case open, you may want to disconnect the annoying buzzer by clipping its leads close to the p.c. board. Then, you can put the whole receiver back together again. Mine seemed more sensitive than it was before I repaired it, but this could have been my imagination.
Good luck and 73 DE AA1LL. The DX-390 is a handy reciever; I use mine mostly for portable c.w. and SSB utility and amateur monitoring. I also use it as a second station receiver to see what bands are active and/or full of QRM (this may be what blasted the receiver as on some frequencies I have transmitter r.f. running around the radio room which is on the second floor).
Paul Gili, AA1LL
5 White St.
Greenville, NH, 03048